In radiotherapy, radiation dosages are typically defined in terms of the energy absorbed per unit mass of tissue. However, relating the prescribed physical dose to the biological effect the radiation will have on the actual tissue being treated is not straightforward. FIG. 1, for example, illustrates these potential dose-response curves 100 indicating the surviving fraction of cells during a treatment protocol versus the administered dose. The dose-response curves can vary among patients, and even at various locations or times for a particular patient. The actual dose-response curve for a particular patient and/or organ, however, is typically not well known in-vivo.
Typically, radiotherapy treatment for deep-seated tumors (as well as to some superficial organs such as the skin) is delivered in a number of fixed sessions, or fractions (e.g., one fraction a day for 30 days) and the dosages are prescribed primarily based on physician and/or institutional experience. For a given total dose, the dose-response curve of a fractional scheme is affected, for example, by the effect of DNA repair and biological damage due to ionizing radiation (or by other therapies such as cryotherapy and chemotherapy). In particular, radiation can directly or indirectly cause breaks in DNA strands, which under some circumstances may be repaired, but in other cases may not be, resulting in cell death.
More particularly, two primary types of cell death occur as a result of radiation exposure—mitotic cell death and apoptosis. In mitotic cell death (which may occur at any time following irradiation), damaged chromosomes cause cells to die as they attempt to divide. Apoptosis, or programmed cell death, occurs normally, and although not typically as prominent as mitotic cell death, can also be induced by radiation and correlate with radiosensitivity.
Because cell death occurs at different rates for different patients, cells, tissues, organs and tumors, dose-response curves for any individual treatment can vary significantly. Therefore, it is difficult to determine, a priori, the proper dose that will kill a given patient's tumor without exposing healthy tissue to unacceptable levels of radiation. Further, the effects of each treatment fraction (both immediately following the fraction and prior to a subsequent fraction) can impact the dose-response curve for a particular treatment. What is needed, therefore, is a way to determine the amount (or lack of) damage caused by therapy dosages, thus giving the physician the ability to determine appropriate adjustments to the therapeutic dosages throughout the treatment cycle that account for the effects of previous radiation fractions on an individual patient's anatomy.